I agree - something between 3/4" and 7/8" works very well. Certain longer flakes, if you are folding them, do well in slightly taller, narrower chambers, but right around that 13/16 mark and about 1" to 1 1/4 tall seems to hit a lot of flakes dead on.
My favorite flake pipes (commercial pipes anyhow) are a Stanwell 63 (tall, narrow, somewhat conical bowl) a Peterson 606 pot, a Stanwell HCA II Churchwarden, and a Kaywoodie.